You conduct experiments to change the dna of the flax plant in hopes of breeding a stronger source of linen. you notice that a single change to a gene causes the sequence of protein a to become drastically longer. what could that change be?

there has been a change of the stop codon.

Explanation:

The stop codon is necessary to stop the synthesis of a protein. When protein A becomes longer than it used to be, it is an indication that the stop codon has been changed hence the increased synthesis of protein A.